1. Field of the Invention
The present invention relates to a motor, and more particularly, to a core type motor, which reduces its cogging torque up to the utmost limits, and which controls its torque ripples, caused by a driving current, at the state of almost zero cogging torque.
2. Prior Art
A schematic cross-sectional view of a prior art motor is shown in FIG. 14(a), which essentially comprises two magnetic pieces 1, a magnetic yoke 2 and an armature 3. The magnetic pieces 1, each made of a semi-circular permanent magnet, are fixed to the magnetic yoke 2 and are arranged in circle such that the their magnetic poles facing the the armature 3 are N-pole and S-pole, respectively. The armature 3 is rotatable about a rotating axis 5 and provided with three protruding poles whose outer ends extend a predetermined length in the peripheral direction, respectively. Each of the protruding poles has a coil 4 wound therearound and energized in a known manner. The outer end surface of each of the protruding poles faces the magnetic poles of the magnetic pieces 1 with a gap g. The rotating axis 5, together with the armature 3 and the magnetic pieces 1, are supported by a supporting member (not shown). FIG. 14(b) is a view illustrating the distribution of the magnetic flux density on the magnetic pole surface, in the motor rotating direction.
In such kind of prior art motor, a cogging torque has been a serious problem, because it causes a big problem of torque ripple. For this, several methods have been proposed to reduce the cogging torque. For example, one of them adapts measures to distribute the magnetization of the magnetic pieces 1 sinusoidally in the motor rotating direction, and another one adapts measures to control a magnetized angle of the magnetization of the magnetic pieces 1 (see Japanese Patent Laid-Open No. 61-254,045/1986. However, no method has been successful in eliminating the cogging torque completely.